The present invention relates to a triplet lens system composed of three lenses each having reflective powers and a diffractive surface and an aspheric surface are mounted on pre-set lenses so as to have better image with lower manufacturing cost.
An image reading device generally consists of three glass spherical lens which includes a meniscus lens, a bi-concave lens and a bi-convex lens arranged in a specific sequence. The optical arrangement is called Cook-Triplet system. However, the glass lens system is suffered with high manufacturing cost that refrains from number of elements of glass lenses. The spherical surface of the glass lenses cannot get rid of image aberration when the aperture and the field angle are increased. This limits the resolution of the conventional triplets lens system.
The use of the plastic lenses may be considered as a solution to the above-mentioned problems.
In accordance with the present invention, the lens system comprises three lenses juxtaposed with each other and each of which has refractive power. The first lens is a plastic lens having a front side formed with a convex surface which has a radius of curvature of R1, and a rear side formed with a concave surface which has a radius of curvature of R2, wherein |R1| less than |R2|.
The second lens is a plastic lens having a front side formed with a convex surface. A shape coefficient X2=((R1+R2)/(R2xe2x88x92R1)) is larger than 0.9. When the front side is a convex surface, R1 greater than 0. When the front side is a concave surface, R1 less than 0. When the rear side is a concave surface, R2 greater than 0. When the rear side is a convex surface, R2 less than 0.
The third lens is a glass lens having positive refractive power. An aspheric surface and a diffractive surface are respectively mounted on the plastic lenses.
The primary object of the present invention is to provide a lens system that can be obtained with low manufacturing cost. A diffractive surface and an aspheric surface are mounted on the plastic lenses. By this way, the optical system can be manufactured by plastic lenses which are made by injection molding of an injection machine is considered to decrease the cost, easier to be produced into aspheric surface or diffractive surface.
The other object of the present invention is to provide a lens system made of plastic material which has more controllable factors so as to eliminate the image aberration and chromatic aberration in glass lenses.